<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 200px;
      height: 100px;
      background-color: pink;

      animation-name: change;
      animation-duration: 1s;
      animation-iteration-count: infinite;
    }

    .box:hover {
      animation-play-state: paused;
    }

    @keyframes change {
      from {
        width: 200px;
      }
      to {
        width: 600px;
      }
    }
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>